This invention relates to arc stabilization processes and devices which may be employed, for example, in arc coating systems. Such coating systems are disclosed in U.S. Pat. Nos. 3,625,848 and 3,836,451 to Alvin A. Snaper and U.S. Pat. Nos. 3,783,231 and 3,793,179 to L. Sablev, et al. These systems are characterized by high deposition rates and other advantageous features. However, these advantages can be somewhat offset due to instability of the arc. That is, the arc involves currents of about 60 amperes, or more, concentrated into a cathode spot so small that current densities are 10.sup.3 to 10.sup.6 amperes per square inch. The voltages are 15 to 45 volts. Thus, power densities at the tiny cathode spot are in the order of megawatts/inch.sup.2. Accordingly, local violence is an understatement. The target surface under the cathode spot flash evaporates from the intense heat. It is this evaporated target material which deposits as the coating on a substrate. The cathode spot migrates about the target surface in a random, jerky motion with reported velocities of many meters per second. Because of this random movement, damage to the device and contamination of the coating can occur if the spot moves off the target surface.
Different solutions to the arc instability problem have been proposed. Thus, in Sablev, et al., U.S. Pat. No. 3,793,179, a shield is placed close to the edge of the target. In particular, it is placed at a distance from the target which represents less than a mean free path of the gas present. In an arc discharge, gas and plasma are generated at the cathode spot with sufficient violence that local mean-free-paths may occasionally be reduced to a few thousandths of an inch. When such a blast of local high pressure is blown under the shield, which is spaced at several millimeters (.about.80 thousandths of an inch), there is finite possibility the arc can migrate under the shield. When this happens, there will be arc damage to the cathode, contamination of the evaporant, or the arc will extinguish.
Sablev, et al. U.S. Pat. No. 3,783,231 apparently addresses the foregoing problem by providing a feedback mechanism of some complexity that emphasizes the frustrations caused by the problem. The feedback involves the utilization of a magnetic field to retain the cathode spot on the target surface. U.S. Pat. No. 2,972,695 to H. Wroe also suggests the utilization of a magnetic field for cathode spot retention.
It is an object of the present invention to provide, in an arc coating device, stabilization of the arc in such a manner as to avoid the inadequacies and complexities of the prior art approaches.
Generally, this is effected by surrounding a predetermined area of the evaporation surface of the target with a confinement ring which contacts the target and directs the arc back to the evaporation surface whenever it wanders onto the confinement ring surface even if the ring is coated with conductive material evaporated from the target. As will be described in further detail hereinafter, materials from which the confinement ring may be fabricated are such that the ring is characterized by (a) a low absolute value of secondary electron emission ratio where preferably the ratio of the target is greater than that of the ring and (b) a low surface energy of the confinement ring relative to that of the evaporant.
Although the theory of operation is not completely understood, it is thought the above characteristics function in the following manner to effect the advantageous results of the present invention. Due to the confinement ring's low secondary electron emission ratio, the arc will return to the target whenever the arc wanders onto the ring surface. During the coating process, some of the target material evaporant may deposit on the ring. This could provide a bridge for the arc over the ring in spite of its low secondary emission ratio. However, due to the low surface energy relative to that of the evaporant, there is no wetting of the ring by the evaporant. Thus, the deposit is instanteously evaporated by the arc so that the arc again contacts the confinement ring surface whereby it is returned to the target.
One material which possesses the above characteristics is boron nitride (BN). This material has been used as a wiper and insulator in high current switches. It has also been used as a nozzle on arc spray devices. In such devices, the refusal of BN to permit arcing against its surface is involved. However, in neither case is the BN heavily coated by target material in its operation as discussed above. Instances where BN has been employed for other purposes are disclosed in U.S. Pat. Nos. 3,202,862; 3,555,238; and 3,945,240.
In summary, a primary purpose of this invention is to provide an improved apparatus and method of arc stabilization which provides long term stability in a straightforward manner and which may be used in such applications as arc coating.
Other objects and advantages of this invention will be apparent from a reading of the following specification and claims taken with the drawing.